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生物质和化石燃料在缺氧情况下不完全燃烧产生的黑碳,在地球气候系统中具有独特而重要的作用。因为大气中所含的黑碳会吸收太阳辐射,影响地气系统能量交换,并加快冰雪的融化速度[1-2]。黑碳质量浓度过大会显著影响地区内硫酸盐的冷却效果[3],降低大气能见度[4],直接或间接影响着气候的变化。为应对当前的全球气候变化,我国作出了“2030年碳达峰、2060年碳中和”的承诺[5],因此,目前对黑碳的光学性质的测量及表征具有重要的现实意义。
黑碳(BC)的光学性质比较复杂,为了准确测量大气中真实BC含量, 国内外学者也提出了多种测量方法[6-12]。测量黑碳质量浓度方法主要分为滤膜法和原点位方法[13-14]。目前,滤膜法测量仪器包括滤膜式黑碳仪(AE-33)[15]、OC/EC分析仪等。原点位方法测量仪器包括光声消光仪(PAX)[16]、单颗粒黑碳光度计(SP2)[17]和腔衰减相位移反照射率监测仪(CAPS)[18]。黑碳仪作为最常用的仪器,具有操作简单、便于维护且不受外界气体干扰的优点,但其准确性受到滤膜负载效应和滤膜多重散射效应的影响。以往的研究仅仅局限于使用经验公式计算滤膜负载系数和滤膜多重散射系数[6-11],因此,如何选用准确的技术路线来监测大气中黑碳的质量浓度成为目前的研究焦点。
本研究以常州市滆湖地区为例,在已有研究[10-11]的基础上,使用2种经验公式进行滤膜负载修正系数的计算,然后对滤膜负载修正后的滤膜衰减系数进行拟合,并分析3种波长下的拟合系数;在拟合系数较好的基础上,以PAX作为参考仪器,对滤膜负载修正后的滤膜衰减系数和PAX测量的吸收系数进行拟合,得到多重散射系数Cref。修正后的监测技术路线可以正确地监测大气中黑碳的质量浓度。本研究以常州市滆湖地区监测数据为代表,可为监测区域黑碳气溶胶的管理提供参考。
大气黑碳监测技术改进及系数修正——以常州市滆湖地区监测数据为例
Atmospheric black carbon monitoring techniques improvement and coefficient correction ——An example of observed data of Gehu area in Changzhou
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摘要: 为准确监测大气中黑碳的质量浓度,提高监测数据的代表性和准确性,采用黑碳仪(AE-33)对常州市滆湖地区进行了监测;参照同一时间段另一台光声消光仪(PAX)仪器测得的数据并进行比对,以滤膜负载补偿系数和多重散射系数修正为基础,根据实际测量数据分析波长吸收指数(
$ {a}_{\mathrm{A}\mathrm{b}\mathrm{s}} $ )和二次棕碳占比情况。结果表明:当修正滤膜负载补偿参数时,在520、660和880 nm波长下,拟合系数R2分别为0.988、0.988和0.985,验证了使用2种修正经验公式计算滤膜负载系数的方法具有较高的可靠性;在此基础上,当修正多重散射系数Cref时,2台仪器测量的数据经过拟合得出多重散射系数的值分别为3.706和3.862;2台仪器的质量浓度拟合系数R2为0.96, 误差范围为3%;在常州地区,棕碳和黑碳,这2种物质的吸收指数接近1左右,由此可判断污染源均来自交通排放;一次棕碳与二次棕碳,这2种物质吸收贡献值相比,二次棕碳占主导地位,二次气溶胶的排放量大。综上所述,黑碳仪结合经验公式和数据比对的修正技术路线具有可行性和适用性,能准确测量大气中的黑碳质量浓度,并对环境污染状态做出正确评估。本研究结果可为大气黑碳气溶胶的监测和管理提供参考。Abstract: In order to accurately monitor the mass concentration of black carbon in the atmosphere and improve the representativeness and accuracy of the observed data, the aetholometer (AE-33) was used to monitor the Gehu area in Changzhou. In comparison with the data monitored by another photoacoustic extinction instrument(PAX) during the same period, the absorption Ångström exponent ($ {a}_{\mathrm{A}\mathrm{b}\mathrm{s}} $ ) and the proportion of secondary brown carbon were analyzed according to the actual measured data based on the filter membrane load compensation coefficient and multiple scattering coefficient correction. The results show that when the filter membrane load compensation parameters were modified, the fitting coefficients R2 were 0.988, 0.988 and 0.985 at 520, 660 and 880 nm wavelengths, respectively, which proved the high reliability of the two modified empirical formulas for calculating the filter membrane load coefficients. On this basis, when the multiple scattering coefficient Cref was modified, the values of the multiple scattering coefficient determined from the regression of measured data by the two instruments were 3.706 and 3.862, respectively. The fitting coefficient R2 of mass concentration of the two instruments was 0.96 and the error range was 3%. The absorption Ångström exponent of brown carbon and black carbon in Changzhou were close to 1, so it could be concluded that the pollution sources were from traffic emissions. Compared with and absorption contribution value of first and secondary brown carbon, secondary brown carbon played a dominant role and secondary aerosol emission was large. In conclusion, the modified technical route combined with empirical formula and data comparison is feasible and applicable, which can accurately measure the mass concentration of black carbon in the atmosphere and correctly evaluate the state of environmental pollution .The results of this study can provide a reference for monitoring and management of atmospheric black carbon aerosols. -
表 1 滤膜负载影响因子k值随波长变化趋势
Table 1. Variation trend of k value of filter membrane load with wavelength
波长序号 波长/nm k的平均值 1 370 0.004 13 2 470 0.004 10 3 520 0.003 95 4 590 0.003 66 5 660 0.003 17 6 880 0.001 62 7 950 0.001 26 -
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